There is an increasing requirement in data and multimedia services for more and more local memory in hand-held or mobile devices. Presently, flash memories are the memory type typically used in mobile terminals such as mobile telecommunications devices. As data requirements increase, progress has been made to increase the capacity and decrease the cost of flash memories. However, there is still a need for a data storage medium having low cost, high storage density and high access speed.
Recently, probe storage systems have been developed. Probe storage utilises atomic force microscopy probes having tips which are heated so that when they make contact with a polymer surface of a recording medium, the heated tip softens the polymer surface and creates an indentation or “pit” in the polymer surface.
The probes are used for reading by exploiting the temperature dependent resistance of the probes. The probes are heated to a temperature lower than that required to melt the polymer. When the probe travels into a pit the heat transfer between the polymer and the probe is more efficient and the probe's temperature and hence resistance will decrease. The decrease in resistance is detected to detect the presence of the pit.
More recently developed probe storage devices, such as those shown in “Millipede—A MEMS-Based Scanning-Probe Data-Storage System” by E. Eleftheriou et al, IEEE Transactions on Magnetics Vol. 39, No. 2, March 2003; US 2003/0218960 or US 2004/0047275 use a storage medium and a probe array, wherein either the storage medium or the probe array is scanned in x-y scanning directions. For example, the storage medium may be spring-mounted and can be pulled in the x and y direction by actuators on each edge. The storage medium moves below a two-dimensional array of fixed read/write probes. To access data, the medium is first pulled to a specified location. In addition, a feedback controlled z approaching scheme brings the probe array into contact with the storage medium. This contact is maintained and controlled while x-y scanning is performed for read/write. The array of probes, which may comprise thousands of probes, work simultaneously and each probe writes and reads information in its defined area. The probes thus scan their associated fields of the storage medium in parallel so that high data rates can be achieved. Already, such probe storage prototypes are demonstrating storage density as high as 3 Tb/inch2.
To erase the indentations, the “pile-up” phenomenon is exploited. When bits are written, raised rings of displaced polymer are formed around the indentations. To erase a bit, the probe is positioned in the vicinity of the indentation and injected with sufficient current to cause melting of the polymer. Thus, the previously existing indentation is filled with melted polymer.
However, over time with multiple cycles of writing and erasing, the surface roughness of the polymer increases. The probe tips also become worn and lose sharpness, and this is made worse by increasing surface roughness. Ambient conditions such as humidity can further increase wear of the probe tips. All these effects mean that probe storage devices are extremely sensitive to the number of reading, writing and erasing cycles.
U.S. Pat. No. 6,850,443 discloses a wear levelling algorithm for use with flash memory devices. This is based on counting of the number of times each portion of the device is written and erased, and distributing use so that portions of the device do not become over-used and likely to become unusable before others.
However, probe storage devices are even more sensitive to wear and so a more sophisticated method of determining wear is required.